Field of the Invention
The present invention relates to plate-shaped or box-shaped resin molded articles. More specifically, the present invention relates to a plate-shaped or box-shaped resin molded article which is formed by injection molding and required to have a good appearance.
Description of the Related Art
An example of a plate-shaped or box-shaped resin molded article includes various projecting or recessed portions. For example, a projecting or recessed portion such as a rib-shaped projecting portion for reinforcing or positioning the resin molded article or a stepped portion formed by an ejector pin may be formed on a non-exterior surface of the resin molded article. In addition, a projecting or recessed portion such as an arrow-shaped portion for introducing a product function to the user may be formed on an exterior surface of the molded article.
Here, the exterior surface is a surface that is visible to the user and therefore required to have a good appearance, and the non-exterior surface is a surface at the side opposite to the exterior surface.
In an injection molding process, resin is injected into a cavity of a mold through an injection gate. The flow of the resin is disturbed at the above-described projecting or recessed portion. Accordingly, there is a possibility that a non-uniform section, which is a section having a color different from that of other sections, will be formed in the exterior surface at a position corresponding to the projecting or recessed portion on the non-exterior surface, or at a position near the projecting or recessed portion on the exterior surface.
Japanese Patent Laid-Open No. 2001-277303 proposes a technique for reducing the occurrence of color difference on the exterior surface, in particular, jetting. More specifically, a molding method is proposed which uses a mold in which a resin-flow-velocity reducing portion having a projecting or recessed shape is formed in a cavity for forming a non-exterior surface at a position where the resin flow velocity tends to increase.
According to the method described in Japanese Patent Laid-Open No. 2001-277303, defects on the exterior surface due to excessive resin flow velocity, such as jetting, can be reduced. However, the formation of the non-uniform section due to disturbance of the resin flow cannot be satisfactorily suppressed. According to Japanese Patent Laid-Open No. 2001-277303, an irregular pattern including V-shaped grooves is proposed as the resin-flow-velocity reducing portion on the non-exterior surface. With this structure, when each V-shaped groove is cut along a longitudinal direction thereof, the width of the V-shaped groove is equal to the width of the resin molded article in the direction in which the V-shaped groove is cut. In addition, the height of each V-shaped groove is constant in the longitudinal direction of the V-shaped groove. Accordingly, disturbance of the resin flow occurs only in the thickness direction of the resin molded article, and the resin flow cannot be sufficiently disturbed. As a result, a non-uniform section appears on the surface of the molded article.
As another method according to the related art, the mold temperature and the resin temperature may be increased. However, when the mold temperature is increased, a long cooling time is required. Therefore, the molding cycle time increases and the productivity decreases. Also when the resin temperature is increased, the molding cycle time increases and the productivity decreases.